In optical disc devices in which optical discs such as DVDs (digital video discs) are used as recording media, for optical pick-up devices thereof, pick-up lenses produced by molding glass or plastics are used. Over recent years, in optical discs, blue wavelength is used, and thereby in pick-up lenses, severe lens shape accuracy in the order of 0.1 μm is required. Thereby, even when molding is carried out using a die produced based on a lens design value, a desired lens shape or optical performance may not be realized. This fact results from occurrence of thermal contraction of a lens material and a die. Therefor, methods of die production in which these factors are considered in advance have been proposed.
For example, a method is proposed in which the amount of deviation from a design value of the spherical aberration of a provisional lens molded using a tentative die produced using a design value is detected and then with respect to the amount of deviation, a table, in which of design values of dies, the relationship between a high-order aspherical constant and a spherical aberration amount generated thereby is determined in advance, is referred to and of corresponding aspherical constants, the amount of small change of high-order term is added to an aspherical constant as an adjusting value to design a final molding die (refer to Patent Document 1).
In the method disclosed in Patent Document 1, it is necessary to prepare, in advance, a table in which the relationship between the amount of small change of high-order term regarding an aspherical constant of an aspherical expression and the amount of variation of a spherical aberration value is determined. However, to enhance the accuracy of this table, it is necessary to fabricate a large number of dies and thereby to press a large number and a wide variety of lenses to measure optical performance.
To solve such problems, a method is proposed in which the wavefront aberration of a first tentative optical element molded using a tentative die produced using a design value is measured and then a correction wavefront aberration so as to offset the wavefront aberration is calculated to design a second tentative optical element in which the shape is optimized so as to have the correction wavefront aberration; and based on the shape of the second tentative optical element, a regular die to mold regular optical elements is designed (refer to Patent Document 2).
Further, the method disclosed in Patent Document 1 cannot respond to an unexpected aberration which is not provided in the table.
Therefore, a method is proposed in which a lens is molded using a provisional die produced using a design value, and when the spherical aberration of the thus-molded lens has deviated from a predetermined value, the amount of deviation is adjusted by lens thickness (axial thickness) (refer to Patent document 3).